Vapor grown carbon fiber is produced through thermal decomposition of an organic compound in the presence of fine particles of Fe, Ni, etc., which serves as a catalyst. Such fine particles of Fe, Ni, etc. are typically generated through thermal decomposition of an organo-transition metallic compound. An aromatic compound such as benzene, which is typically used as a raw material of carbon fiber, is fed to a carbon fiber production furnace together with a carrier gas such as hydrogen. The resultant carbon fiber filaments are quite minute, since they are grown in the production furnace within a very short period of time.
Conventionally, vapor grown carbon fiber has typically been produced by means of a method in which an organic compound, an organo-transition metallic compound, and a carrier gas are fed directly to a carbon fiber production furnace which has been heated to a predetermined temperature. However, various improved methods for producing carbon fiber have replaced this method.
For example, there has been proposed a method in which an organo-transition metallic compound is decomposed in a thermal decomposition zone, the decomposition zone being separated from a carbon fiber production zone, and the resultant decomposed gas is introduced into the carbon fiber production zone; a gas of an organic compound and a carrier gas are fed to the production zone, to thereby thermally decompose the organic compound; and carbon fiber is grown in the production zone in the presence of fine particles of Fe, Ni, etc. serving as a catalyst, which have been generated through thermal decomposition of the organo-transition metallic compound (Japanese Patent Publication (kokoku) No. 6-21377).
There has also been proposed a method in which an organic compound solution in which an organo-transition metallic compound is dissolved is gasified in the presence of a carrier gas, to thereby produce a gas mixture in which the ratio of the organo-transition metallic compound to the organic compound is equal to the corresponding ratio of these compounds in the solution; and the gas mixture is caused to react at a high temperature in a heating zone, to thereby produce vapor grown carbon fiber (Japanese Patent Publication (kokoku) No. 4-13448 (U.S. Pat. No. 4,572,813)).
Vapor grown carbon fiber, by virtue of its small diameter, is employed as a filler used in composite materials such as resin- or rubber-based composite materials; and as a semiconductor material; a catalyst material; and a field emission material. Vapor grown carbon fiber desirably has a uniform and very small outer diameter. Carbon fiber is generally grown around fine particles of Fe, Ni, etc. serving as a catalyst (i.e., nucleui). Therefore, in order to produce carbon fiber of uniform outer diameter, firstly, fine particles of uniform size must be formed. Secondly, variation in carbon fiber production conditions should be minimized in a carbon fiber production zone, the production conditions including the concentration of fine particles, the concentration of a product obtained through decomposition of an organic compound, temperature, and the residence time of the decomposed product in the carbon fiber production zone.
Fine particles of a transition metal such as Fe or Ni which are generated through decomposition of an organo-transition metallic compound become large with passage of time. Therefore, in order to produce fine carbon fiber, the residence time of the fine particles in a carbon fiber production zone must be reduced, to thereby prevent growth of the fine particles. When the residence time of the fine particles is long, the particles may become large and prevent production of carbon fiber.
In general, since a carbon fiber production furnace is heated through application of heat from the outside, maintenance of uniform temperature inside the furnace is difficult. Particularly when the diameter of the production furnace (reactor tube) is increased in order to increase productivity of carbon fiber, elevating the temperature of a raw material gas, etc. requires a long time, and the temperature of the inside of the furnace tends to differ from portion to portion.
Measures proposed by the aforementioned publications, Japanese Patent Publication (kokoku) Nos. 6-21377 and 4-13448 (U.S. Pat. No. 4,572,813) to solve the above problems still require some improvements.
Accordingly, the first object of the present invention lies in obtaining carbon fiber having a substantially uniform outer diameter by means of a method in which a raw material gas is mixed with a carrier gas which has been heated to high temperature, and the resultant gas mixture is fed to a carbon fiber production furnace (carbon fiber production zone), whereby both the time required for raising the temperature of the raw material gas in the furnace and the residence time of the raw material gas in the furnace are shortened, and uniform carbon fiber production conditions in the furnace are attained.
As above described, the vapor grown carbon fiber is produced through thermal decomposition of an organic compound in the presence of fine particles of a transition metal such as Fe or Ni, which serve as catalyst. The formation mechanism of carbon fiber is as follows: carbon generated through thermal decomposition of an organic compound is deposited around fine particles of a transition metal, and carbon fiber is formed through extension of the resultant network of carbon atoms. Therefore, in order to produce fine carbon fiber, the fine particles must be reduced in size. Production of fine carbon fiber can be attained by shortening growth time, while employing very fine particles of a transition metal.
Vapor grown carbon fiber is used, as an electrically conductive filler or a thermally conductive filler, in synthetic resins, paints, lithium batteries, etc. Furthermore, vapor grown carbon fiber is used in, among other materials, electrode materials of a fuel cell, a secondary battery, a capacitor, etc.; and electron emission materials of a field emission display (FED). Vapor grown carbon fiber used in such materials desirably has a very small diameter.
Conventionally, there have been proposed various methods for causing fine particles of a transition metal to be present in a carbon fiber production furnace (a reaction furnace).
For example, there has been proposed a method in which a transition metallic compound is thermally decomposed in advance to thereby obtain fine particles of the transition metal, and the resultant fine particles are caused to be present in a reaction furnace (Japanese Patent Publication (kokoku) No. 58-22571).
There has also been proposed a method in which an organo-transition metallic compound having an evaporation temperature (gasification temperature) lower than its thermal decomposition temperature, such as ferrocene, is gasified and fed into a reaction furnace, and the gasified compound is thermally decomposed in the furnace, to thereby generate fine particles of the transition metal (Japanese Patent Publication (kokoku) No. 62-49363 (U.S. Pat. No. 4,572,813)).
There has also been proposed a method in which a transition metallic compound is dissolved in a solvent, the resultant solution is sprayed into a reaction furnace, and the compound is decomposed in the reaction furnace, to thereby generate fine particles of the transition metal (Japanese Patent No. 2778434).
Very fine particles of a transition metal are difficult to obtain through the aforementioned conventional techniques. The size of fine particles of a transition metal obtained through pulverization has its limit of the minimization, and the particles tend to aggregate to form secondary particles; i.e., the particles increase in size. Meanwhile, even when fine particles of a transition metal are obtained through gasification and thermal decomposition of an organo-transition metallic compound such as ferrocene, the resultant particles tend to aggregate in the range of thermal decomposition temperature, and the particles increase in size.
When a transition metallic compound is gasified and then fed into a reaction furnace, the compound must be evaporated at a temperature lower than its thermal decomposition temperature. Therefore, transition metallic compounds which may be employed are limited. A compound such as ferrocene satisfies the above requirement; i.e., the evaporation temperature must be lower than the thermal decomposition temperature, but involves economical problems due to its high cost. When a transition metallic compound is gasified and then fed into a reaction furnace, variation in size of the resultant particles tends to occur. In this case, since the compound is fed into the reaction furnace while assuming a molecular state, thermal decomposition of the compound occurs easily, and most of the compound is thermally decomposed in the vicinity of the inlet of the furnace. The resultant metallic particles aggregate while flowing in the gas, and become too large to serve as catalyst. Therefore, the production efficiency (yield) of fine carbon fiber is low.
A method in which a transition metallic compound is dissolved in a solvent, the resultant solution is atomized into fine droplets, and the resultant droplets are fed into a reaction furnace is advantageous, in that a wide variety of transition metallic compounds can be employed by selection of an appropriate solvent. However, the method involves a problem that carbon fiber of uniform diameter cannot be produced, since an employed solvent is evaporated in a reaction furnace, and the temperature of the interior of the furnace cannot be maintained uniform, due to the effect of latent heat of evaporation of the solvent.
Accordingly, the second object of the present invention is to provide a method for efficiently producing fine vapor grown carbon fiber, in which a wide variety of transition metallic compounds, serving as catalyst, can be employed.